Balancing the redox reaction for Cu and dilute HNO₃ Steps:

• Oxidation: Cu → Cu²⁺ + 2e⁻ (×3: 3Cu → 3Cu²⁺ + 6e⁻)
• Reduction to NO: NO₃⁻ + 4H⁺ + 3e⁻ → NO + 2H₂O (×2: 2NO₃⁻ + 8H⁺ + 6e⁻ → 2NO + 4H₂O)
• Combine ions: 3Cu + 2NO₃⁻ + 8H⁺ → 3Cu²⁺ + 2NO + 4H₂O
• Include 6NO₃⁻ for 3Cu(NO₃)₂; total 8HNO₃ provides H⁺ and NO₃⁻ Why D is correct:
• Balanced equation 3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 2NO + 4H₂O requires 8 mol HNO₃ per 3 mol Cu by electron transfer balance. Why the others are wrong:
• A. 2 moles: Underestimates NO₃⁻ for both reduction and Cu(NO₃)₂ formation.
• B. 4 moles: Fits single Cu with NO₂ production, not 3Cu or NO.
• C. 6 moles: Covers only NO₃⁻ for products, ignores 2NO₃⁻ reduced to NO. Final answer: D